ISDN (Integrated Services Digital Network) and ISUP (ISDN User Part—part of Signalling System no. 7, SS7) are sets of communication standards for making calls in a telecommunications network. ISUP and ISDN calls involve two key elements: an exchange of signalling messages to set up and tear down the call, and media for carrying the voice or data (e.g. fax) traffic.
The signalling for these protocols is described as out-of-band. That is, the signalling and media do not use the same TDM timeslot (DS0/E0). Media is carried in a unique DS0 (E0) channel but the signalling is carried separately. In the case of ISUP and NFAS (Non-Facility Associated Signalling) ISDN PRI, the signalling may use a different TDM carrier (T1/E1) altogether, even a different Media Gateway. A TDM carrier includes a number of separate DS0 or E0 channels. In the case of SS7 (Signalling System 7), rather than transporting the signalling over TDM, it is transported out of band over IP using M2PA (Message Transfer Part 2 Peer-to-Peer Adaptation Layer) or point-to-point M3UA (Message Transfer Part 3 User Adaptation Layer).
FIG. 1 shows an example of the connections in a typical SS7 network. The signalling messages are typically provided to and utilised by a Feature Server or Media Gateway Controller. A Media Gateway Controller is used to control Media Gateways. It also has some other signalling (ISUP, ISDN, etc.) that it uses to work out what media paths to set up. A Feature Server would include the function of a Media Gateway Controller but it would also have capabilities to provide subscriber call features (e.g. speed dial functionality). In the present application references to Media Gateway Controller refer to either a Media Gateway Controller or a Feature server providing that functionality. Furthermore, it may also encompass its role as a Signalling End Point in an SS7 network, rather than just a Signalling Transfer Point (STP).
This Feature Server/Media Gateway Controller determines how to set up a call and uses a control protocol such as H.248 to program the necessary connectivity and attributes to a Media Gateway which terminates one end of the media TDM carrier to establish the media path. The signalling exchange includes identifiers understood by the Media Gateway Controllers at each end that ultimately map to the TDM timeslots used for media. Where the signalling is transported over TDM, it is transported through, but not consumed by, a Media Gateway.
Diagnosing problems with a call can require analysis of both signalling and media information which, as described above, can involve several hardware elements, each arranged such that they only have partial knowledge of the overall position.
In addition, when there has been a problem with a call, the report of the problem usually only includes minimal details, such as the called/calling number and time of call.
Both of these issues present a problem that would usually require piecing together configuration and traces from several network elements to attempt to provide a consolidated view of all aspects of call setup and tear down needed for diagnosis of the causes and remedies of a specific problem. For example, a Media Gateway Controller (MGC) has configuration correlating ISUP Circuit Identification Codes (CICs) or ISDN Channel Identifiers with particular TDM timeslots on a particular Media Gateway, e.g. via the H.248 TerminationId parameter.
However, when a failed call is reported, and an attempt to analyse the causes and issues with the call, only limited access to parts of the network may be available and it may not be possible to access all of the information about a call. The analysis may be based on only a small portion of the network and so only information within the scope of that analysis will be available. The scope of the monitored network may only include one of two elements and might not include any MGCs at all, and so the potential diagnostic information in the MGCs might not be available. This can make it difficult to provide useful diagnostic information based on the passage and progress of a call in a part of a network excluding an MGC.